1. Field of the Invention
This invention relates to turbine tip seals and especially to reducing leakage losses at the turbine wheel tips while increasing rotor stiffness.
2. Description of the Prior Art
Current tip seal designs for turbine wheels each have their own unique benefits and detriments. Straight, smooth inner bearing surfaces for turbine tip seals traditionally have been used. Straight seals provide a high amount of rotor stiffness; however, these seals also have a high rate of fluid leakage. The low cost of these seals makes them commercially attractive.
A second type of tip seal commonly used includes a honeycomb or cellular inner bearing surface. The cellular characteristics of this structure provides a tortuous, turbulent fluid path and hence a low fluid leakage rate; however, these cellular characteristics also result in a low amount of rotor stiffness. A primary benefit of employing honeycomb seals is that during operation of the turbine, in an out-of-balance condition, the turbine runners rub against the seal and the cells remain capable of performing the sealing function without catastrophic results. High manufacturing costs are associated with the fabrication of honeycomb tip seals because a brazed corrugated ribbon is used to form the honeycomb. The structure is then centrifugally brazed to a substrate material to prevent the honeycomb cells from being filled with brazing material.